These reactors comprise a reaction chamber in which gases and solids react, a centrifugal separator, and one or more heat exchangers for adjusting the temperature of the reaction chamber.
Boilers comprise a combustion chamber in which fuel is burned, a centrifugal separator, and one or more heat exchangers for adjusting the temperature of the reaction chamber.
For simplicity, the prior art described in relation to the present application covers only circulating fluidized bed boilers.
The fuel circulates in a fluidized bed consisting of particles in suspension in air. Fluidization entrains the particles toward the top of the combustion chamber or the reaction chamber, from which they are evacuated to a circular section centrifugal separator which separates the particles from the flue gases. The speed of the flue gases is from 3 m/s to 6.5 m/s in the combustion chamber and from 4 m/s to 6.5 m/s on the axis of the separator. The solid particle content of the flue gases can be as high as 20 kg/Nm3 and the particle size of the circulating particles is less than 500μ.
The centrifugal separator comprises a vertical vortex chamber which has vertical walls, one or more inlet orifices in the upper portion of the separator which receive flue gases to be purified, one or more evacuation orifices for purified flue gases, and one or more evacuation orifices for separated particles in the lower portion of the separator and connected to the bottom of the combustion chamber. The evacuation orifice for purified flue gases is in the upper portion of the separator, i.e. above the area in which the particles are separated.
The walls of the separator converge toward the bottom in order to channel the captured particles toward the lower evacuation orifice. This lower portion is of conical shape, as appropriate to the shape of the separator.
Some of the captured particles are cooled by passing them through a parallel cooling circuit and fed back into the bottom of the combustion chamber or the reaction chamber, where they begin a new cycle in order to maintain a fluidized bed in the combustion chamber or the reaction chamber, and the remaining particles are fed back directly into the bottom of the combustion chamber or the reaction chamber. This circuit constitutes the reaction chamber solids loop.
To reduce the amount of SO2 emitted, limestone particles are introduced into the circulating fluidized bed. However, these particles are only partly sulfated on each pass through the combustion chamber or the reaction chamber. It is therefore necessary to ensure that they remain in the circulating fluidized bed for as long as possible.
The flue gases are evacuated to the atmosphere after passing through a series of heat exchangers in a cage to the rear of the boiler, in which they are cooled.
The temperature of the combustion chamber or the reaction chamber can be controlled by fluidized bed exchangers situated in tubed or untubed exterior beds and which can be contiguous with the bottom of the combustion chamber or the reaction chamber. The exchangers situated in the combustion chamber or the reaction chamber are L-shaped or U-shaped exchangers and/or omega tube panels.
The particles that circulate in the reaction chamber solids loop cause serious erosion of the walls of some parts of the circuit, such as the bottom of the combustion chamber or the reaction chamber, the separator, the inlet duct and the solids return duct, which makes it necessary to cover the walls with a thick refractory material. This leads to a significant increase in the manufacturing cost of the boiler and a significant increase in the suspended weight. The increase in the suspended weight makes it essential to provide reinforced frameworks to support these components. The refractory materials have a high thermal inertia which increases the time to warm up the boiler and cool it down on starting and stopping it.
It is also possible to make some walls of the separator from parallel tubes joined together by fins, the tubes carry a heat-exchange fluid such as water and/or steam and thus constitute cooling surfaces. The walls cooled in this way reduce the thickness of the refractory material layer needed. However, producing these walls for a circular geometry separator is complex and costly. In fact, circulating water in the walls necessitates a multitude of feeder and evacuation tubes and connectors.
Moreover, the arrangement of the reaction chamber solids loop with independent elements interconnected by ducts in which solids or gases circulate leads to a high weight and bulk.
Attempts have therefore been made to optimize this type of installation by using plane walls for a distinctly non-circular section centrifugal separator, as in the patents EP 481 438 and EP 730 910. This solution enables the use of a thin refractory material layer on the wall of the separator and therefore reduces its weight. This solution also creates a module that can be reproduced if the power of the installation is to be increased. However, this type of solution is not satisfactory because the flue gases enter the centrifugal separator via an orifice that does not enable sufficient acceleration of the particles and the gases contained in said flue gases. Their speed being insufficient, the particles are difficult to separate in the separator. This leads to loss of particles from the bed in the flue gases evacuated via the outlet of the separator. This is highly unfavorable from the point of view of heat transfer in the combustion chamber, the rate of sulfation of the fine limestone particles in the combustion chamber, and the oxidation of the fine carbon particles in the combustion chamber or the reaction chamber. The fine particles are discharged into the atmosphere before virtually total sulfation or oxidation.
It was then proposed, as in the applicant's application EP 01 402 809.6, to use the walls of the rear cage as common cooling walls for the combustion chamber or the reaction chamber, on the one hand, and the centrifugal separator, on the other hand, in order to be able to place an acceleration duct between the combustion chamber or the reaction chamber and the separator. This assembly constitutes a basic module. The acceleration duct accelerates the flue gases from 15-20 m/s at the duct inlet to 25-35 m/s at the duct outlet, so that the solid particles can be accelerated in order to separate them better centrifugally and to cause preseparation of the particles contained in the flue gases on the walls of the duct. Another fundamental feature is the pyramidal shape of the lower portion of the separator, this truncated pyramid shape preventing rebounding of the vortex flow of flue gases on one of the walls of the lower portion. However, this configuration of the basic module makes it obligatory to place the combustion chamber (or the reaction chamber), the separator and the rear cage at right angles, the separator having a common wall with the rear cage. If it is required to increase the power of the installation, it is necessary to increase the number of separators, and this form of the basic module rules out simple production of assemblies comprising odd numbers of separators, starting from three separators.